An emerging model of action for steroid-binding proteins is that they are essential in delivering steroids to either steroid membrane or cytoplasmic receptors. Whereas the more commonly known model suggests that steroid-binding proteins such as sex hormone binding globulin (SHBG) are only made in the periphery and only deliver steroids to be passively released at the target cell, we have found that SHBG is produced in the brain where it facilitates female sexual receptivity. Therefore, we have two ends of the answer to the question of whether SHBG is a neuromodulator: its production and its behavioral effects. However, we need to find out whether and where there are receptors for SHBG in brain. In generating preliminary data, we have found that SHBG stimulates female sexual receptivity and that coupling it to dihydrotestosterone blocks this facilitative effect. We have found that microiontophoresing SHBG onto hypothalamic tissue sections results in an immediate depolarization of magnocellular neurons. We have found that cells transfected with either estradiol receptor alpha or beta respond to both SHBG and SHBG-estradiol application with an elevation of MAP kinase phosphorylation. Using SELDI-TOF mass spectrometry on affinity chromatography eluted hypothalamic tissue, we have identified protein peaks at 144000 and 168000 daltons that seem to represent SHBG receptors. We have found SHBG produced in hypothalamic areas in the same neurons as the reproductively important peptide oxytocin. Both SHBG and oxytocin are found in varicosities and in synaptic vesicles suggesting they are released within the brain. Therefore, work done in several laboratories, collaborating together via the PI, has found that SHBG is made in brain, may be released in brain, [and] has important very rapid actions on neurons and cells in vitro, and so we would like funding to search for receptors for this potentially very important brain protein. This application proposes a confluence of particular scientific expertise including the PI and Drs. Gustav Jirikowski at the University of Jena, Germany, Jeffrey Tasker of Tulane University and Robert Shapiro at the Oregon Health Sciences University. In the words of one of the reviewers of the initial application, "The mechanism mediating the delivery of steroid hormones to their receptors has important implications for mental health, as well the action of steroid hormones in cancer, heart disease, and other disorders." This proposal examines this mechanism.